Influence of Hyperinsulinemia and Insulin Resistance on In Vivo β-Cell Function

OBJECTIVE Recent work has shown that insulin stimulates its own secretion in insulin-sensitive humans, suggesting that insulin resistance in the β-cell could cause β-cell dysfunction. We have tested whether insulin exposure and insulin sensitivity modulate β-cell function in subjects with normal glucose tolerance (NGT) and whether they contribute to dysglycemia in impaired glucose regulation (IGR). RESEARCH DESIGN AND METHODS Insulin sensitivity (by euglycemic clamp), insulin-induced secretory response at isoglycemia (IISR) (as C-peptide percent change from basal during the clamp), glucose-induced secretory response (GISR) to an intravenous glucose bolus, and β-cell glucose sensitivity (β-GS) (by oral glucose tolerance test [OGTT] modeling) were measured in 1,151 NGT and 163 IGR subjects from the RISC (Relationship between Insulin Sensitivity and Cardiovascular Disease) study. RESULTS In NGT, IISR was related to both insulin sensitivity and antecedent insulin exposure; GISR was related to insulin exposure. IISR was positively, if weakly, related to β-GS (r= 0.16, P < 0.0001). Both IISR (-23 [39] vs. -9 [2]%, median [interquartile range], P < 0.03) and β-GS (69 [47] vs. 118 [83] pmol ⋅ min(-1) ⋅ m(-2) ⋅ mmol(-1) ⋅ L, P < 0.0001) were decreased in IGR compared with NGT. Insulin sensitivity and β-GS were the major determinants of mean OGTT glucose in both NGT and IGR, with a minor role for IISR. In a multivariate logistic model, IGR was predicted by β-GS (odds ratio 4.84 [95% CI 2.89-8.09]) and insulin sensitivity (3.06 [2.19-4.27]) but not by IISR (1.11 [0.77-1.61]). CONCLUSIONS Pre-exposure to physiological hyperinsulinemia stimulates insulin secretion to a degree that depends on insulin sensitivity. However, this phenomenon has limited impact on β-cell dysfunction and dysglycemia.